After Effect CC2015 running REALLY SLOW previews on El Capitan

Hey there, So I bought this new High end retina iMac, and it cant preview the most simpel animation in real time now.. can anyone tell me if im doing something wrong?

I make a comp, 500x500, 30fps and quarter resolution

then i make simple white circle and rotate it around the middle of the comp, and when i playback i get 26/30 fps(NOT REALTIME)

I have tried changing all the options under 'preview in preferences', i've tried changing adaptive resolution and pretty much tried everything.. my old 2011 laptop on cs4 runs 100% smooth, so why wont this 5k imac with and almost empty ssd?

I have a feeling that the problem lies with El Capitan (osx 10.11). I just upgraded my 2012 Mac Pro to 10.11 last week and immediately noticed a pretty bad UI lag when ram previewing anything, no matter how simple the comp. In my case, if I use the tilde key to full screen just the Comp window, everything plays back super smooth. However, if I have the full UI with the timeline window showing during playback, I get a noticeable stutter in the playback window. It almost looks like it's playing at half speed even though it says "23.976" and "realtime".

I get the feeling that Adobe has a lot of tweaking to do in order for El Capitan to be a dependable OS for those in a production environment. Luckily, I have a backup of my previous OS and I'm reverting back to Yosemite (10.10) until things improve.